As an electronic component having an electrode formed on a silicon semiconductor substrate, solar cell elements disclosed in Japanese Patent Application Laid-Open Publication No. 2000-90734 (Patent Document 1) and Japanese Patent Application Laid-Open Publication No. 2004-134775 (Patent Document 2) have been known.
FIG. 1 is a schematic view showing a general sectional structure of a solar cell element.
As shown in FIG. 1, the solar cell element is structured by using a p-type silicon semiconductor substrate 1 whose thickness is 220 to 300 μm. On a light receiving surface of the silicon semiconductor substrate 1, an n-type impurity layer 2 whose thickness is 0.3 to 0.6 μm, and an antireflection film 3 and grid electrodes 4, which are on the n-type impurity layer 2, are formed.
On a back surface of the p-type silicon semiconductor substrate 1, an aluminum electrode layer 5 is formed. The formation of the aluminum electrode layer 5 is conducted through applying an aluminum paste composition containing aluminum powder, a glass frit, and an organic vehicle by employing screen printing or the like; drying; and thereafter, firing the aluminum paste composition for a short period of time at a temperature greater than or equal to 660° C. (melting point of aluminum). During the firing, the aluminum is diffused into the p-type silicon semiconductor substrate 1, whereby an Al—Si alloy layer 6 is formed between the aluminum electrode layer 5 and the p-type silicon semiconductor substrate 1 and concurrently, a p+ layer 7 is formed as an impurity layer resulting from diffusion of aluminum atoms. The presence of the p+ layer 7 prevents recombination of electrons, and therefore a BSF (Back Surface Field) effect which enhances a collection efficiency of generated carriers can be obtained.
For example, as disclosed in Japanese Patent Application Laid-Open Publication No. 5-129640 (Patent Document 3), a solar cell element in which a back surface electrode 8 including an aluminum electrode layer 5 and an Al—Si alloy layer 6 is removed by using acid or the like and a collecting electrode layer is newly formed by using a silver paste or the like has been put into practical use. However, since disposal of the acid used for removing the back surface electrode 8 is required, for example, a problem that the disposal makes a process complicated accrues. In recent years, in order to avoid such a problem, many solar cell elements have been structured with the back surface electrode 8 left as it is and utilized as a collecting electrode.
In the meantime, in order to reduce costs in manufacturing solar cells, making a silicon semiconductor substrate thinner has been examined these days. However, when the silicon semiconductor substrate is thinner, after firing the aluminum paste composition, a back surface having a back surface electrode layer formed thereon is deformed in a concave manner due to a difference between thermal expansion coefficients of silicon and aluminum, thereby deforming and bowing the silicon semiconductor substrate. As a method for inhibiting the bow, there is a method in which an applying amount of the aluminum paste composition is decreased and the back surface electrode layer is made thinner. However, when the applying amount of the aluminum paste composition is decreased, blisters and globules of the aluminum are easily formed during the firing. Consequently, a fracture or the like in the silicon semiconductor substrate is caused in a process of manufacturing the solar cells, thereby resulting in a problem that manufacturing yields of the solar cells are reduced.
In order to solve the above-mentioned problems, a variety of aluminum paste compositions have been proposed.
Japanese Patent Application Laid-Open Publication No. 2004-134775 (Patent Document 2) discloses, as an electrically conductive paste which is capable of reducing contraction of an electrode film caused during the firing and of inhibiting bow of an Si wafer, an aluminum paste composition which includes aluminum powder, a glass frit, and an organic vehicle, the organic vehicle containing particles which have low solubility or irresolvability, the particles being at least one kind of organic compound particles and carbon particles.
In addition, Japanese Patent Application Laid-Open Publication No. 2005-191107 (Patent Document 4) discloses a method for manufacturing a solar cell element, which attains a high performance back surface electrode in which formation of globules and protrusions of aluminum and blistering of the electrode are inhibited and achieves high productivity by reducing bow of a semiconductor substrate. In an aluminum paste used in the disclosed method, aluminum powder of which mean particle size D50 in cumulative particle size distribution on a volume basis is 6 to 20 μm and in which particles each having a particle size less than or equal to a half of the mean particle size D50 account for 15% or less of all particles in the particle size distribution is contained.
However, even when these aluminum pastes were used, it was impossible to sufficiently inhibit formation of blisters and globules of aluminum in the back surface electrode layer, caused during the firing, and, moreover, to sufficiently reduce bow of a semiconductor substrate.
[Patent Document 1] Japanese Patent Application Laid-Open Publication No. 2000-90734
[Patent Document 2] Japanese Patent Application Laid-Open Publication No. 2004-134775
[Patent Document 3] Japanese Patent Application Laid-Open Publication No. 5-129640
[Patent Document 4] Japanese Patent Application Laid-Open Publication No. 2005-191107